Many architects will be familiar with the Red List of building materials. A key part of the Living Building Challenge, it is a list not so much of materials but of additives and ingredients, each of which has a toxic legacy. Many of the nasties are familiar: asbestos, certain common wood treatments and a slew of volatile organic compounds that off-gas as the “new building smell” we are no longer so enamoured of. Readers of Architecture NZ’s recent article (July/ August 2025) on Claude Dewerse and Mike LeRoy- Dyson’s work helping to deal with 1200 floodcompromised Auckland houses will have noted the troublesome legacy not of Red List items but of seemingly innocuous building elements — they are problematic not so much during the life of the building but at its end.

A key set of difficulties deconstruction crews encounter relates to the removal of concrete slabs. Concrete floor slabs, alongside concrete landscaping and driveways, constitute a huge portion of the material in a house, both by weight and by volume. Concrete on its own is relatively straightforward to recycle. It can be broken up on site before being transported to companies which receive the concrete, crush it and offer it back to the industry as a range of products. A key advantage is that this saves the costs and emissions involved in extracting and crushing fresh aggregate. There are a couple of sweeteners. When re-used, the latent cement content of the recycled aggregate allows a slight reduction in the need for expensive and energy-intensive new cement. Further, recent research has shown that crushed concrete absorbs CO₂ from the atmosphere at a rapid rate; if crushed and left exposed for a few months, it can sequester up to 10 per cent of the concrete’s weight.

Given concrete’s carbon footprint — studies suggest it contributes as much as eight per cent of global greenhouse emissions — architects will, no doubt, seek to minimise their use of the material. For some uses, it is difficult to avoid but we are likely heading towards a time when the ostentatious use of concrete is the architectural equivalent of wearing fur. A still-developing alternative is the use of lowcarbon concrete, which substitutes a portion of the emissions-heavy cement with fly ash or industrial slag. Architects might roll their eyes — it has the feel of claims of ‘low-fat cake’. There are indeed questions but answers are coming. The altered chemistry means that low-carbon concrete takes longer to cure than does regular concrete; that is, its strength is gained more slowly over time. However, careful construction sequencing can usually avoid or mitigate the need for extended construction times. Both fly ash and slag are by-products of burning coal so the success of other carbon-reduction efforts is reducing the available supplies. Alternatives, though, may be surprisingly close at hand; Dr Enrique del Rey Castillo at the University of Auckland is researching the ways in which ‘supplementary cementitious material’ made from natural, locally sourced materials such as pumice, a form of volcanic ash, and kaimoana shells can be used instead of fly ash. And, as with almost every aspect of our construction industry, New Zealand’s small market creates special wrinkles; the market is dominated by concrete suppliers with close financial ties to cement manufacturers, diluting the incentives for concrete suppliers to reduce cement content.

A wildly complicating factor for Dewerse and LeRoy-Dyson’s work is the presence of polystyrene cast into the concrete slab, usually through the very common practice of using polystyrene pods to create waffle slabs. Designers will no doubt be familiar with the issues that polystyrene can cause during construction: most particularly, its susceptibility to blow around the site but, also, that cutting and shaping processes pepper the site with microplastics. More difficult problems emerge during demolition. Dewerse and LeRoy-Dyson have worked with their deconstruction contractors to develop a process for extracting slabs formed with polystyrene. The ideal is to slice up the slab carefully and remove it from the site in large pieces. It is taken apart with a digger, seeking to minimise damage to the polystyrene. A member of the deconstruction team stands ready with a bag to snatch up any crushed pieces of polystyrene, and with a leaf blower on vacuum setting to collect finer particles. This process relies on optimal conditions; a gust of wind can entirely undo these efforts. Where wind issues can’t be avoided, teams enclose the worksite with fencing covered with scrim, providing both shelter and additional containment. As noted in the previous article, the costs of disposing of such a slab might rival the costs of constructing it in the first place.

Polystyrene can be collected for recycling on construction sites. Offcuts must, however, remain clean; the inclusion of soil or other construction residues will lead to rejection by recyclers. For deconstruction crews, the issue is that polystyrene bonds to concrete and is difficult to remove at all, let alone cleanly. As a result, the concrete leaves the site entwined with the polystyrene, which recyclers regard as a contaminant, and usually goes to landfill. One local demolition company does use a crusher equipped with an air lance that blows dust and other lightweight elements out of the aggregate as it is processed. The resulting material is sold for uses such as farm tracks — a win for landfill diversion but, of course, it contains entrained microplastics that will inevitably make their way into the agricultural environment and beyond.

As might be expected, Dewerse and LeRoy-Dyson’s plea to architects is to explore the alternatives: the recycled hollow plastic waffle-slab former systems, such as Cleva Pod, Qpod or Firth’s RibRaft X-Pod. The elements of these systems are easier to transport and, while the material costs are higher than those for polystyrene, there are compensating savings in smaller transport volumes. A key advantage of plastic pod systems comes at the end of the slab’s life when heavy machinery can easily and cleanly reduce the slab into piles of concrete rubble, polythene and steel, ready for recycling. Cleva Pod confirmed the effectiveness of its system by demolishing a trial foundation slab, demonstrating that it eliminates the need for laborious processes or expensive contamination control.

While it is not true of a generation of Christchurch architects who have had the grim duty of witnessing their quake-damaged city being torn down, designers rarely see their creations being demolished. That is, with buildings lasting 50 years or more, we aren’t exposed to the impact of our own design decisions as the construction process runs in reverse. Dewerse and LeRoy-Dyson’s work is giving us a window into the legacy that current practices generate, and they challenge us to respond to what we see.

Architecture NZ and ArchitectureNow will report further on Dewerse and LeRoy-Dyson’s project in upcoming issues, hoping to share the lessons architects might learn.